The present invention relates to a multichannel duplex radio system, typically in the form of portable radio transmitter/receiver equipments such as cordless telephones.
A single channel duplex communication system is known, for example, from British Pat. No. 1,556,087.
In such a system the receiver portion makes use of a so-called "zero IF" or "direct conversion" method of demodulation. Such a method is descirbed in Bristish Pat. No. 1,530,602 and our co-pending British Application No. 8,128,956. Briefly, local oscillator signals in phase quadrature at the carrier frequency are each separately mixed with the incoming audio modulated signal. The resulting signals have zero IF with the two sidebands folded over on each other at the baseband and extending in frequency from DC to the single sideband width of the original signal. The mixer outputs are low pass filtered and then amplified to a standard level. After amplification the two signals are separately differentiated. Each differentiated signal is then multiplied by the input to the other differentiator and one of the multiplier outputs is subtracted from the other.
A system for a full duplex single channel radio transmitter/receiver is disclosed in British Patent Application No. 8,027,566. The "zero IF" or "direct conversion" method of demodulation is again used, but the radio signals are transmitted in digital form.
The transmission is effected by way of frequency shift keyed (FSK) digital signals, the transmitter portion including a source of digital signals and a local oscillator capable of being modulated by the digital signal to produce FSK signals. The local oscillator FSK signals are coupled to a transmission medium. The FSK signals received from the transmission medium are mixed with the local oscillator output to produce quadrature output signals, and a zero IF demodulator to which the quadrature outputs are applied produces a digital output which signal is delayed and applied to a digital network wherein cancellation with corresponding digital signals from a splitter/combiner network is effected.